fuel efficiency changing from small blocks to big blocks?

[Tim.M]

A 6,000 lb boat currently has twin small block GM 350 cubic inch 5.7 liter engines / 260 hp each. It cruises at 3,500 rpm around 35 knots.

If this boat were repowered with twin GM 454 cubic inch 7.4 liter engines / 330 hp each what would you expect to happen to the fuel economy cruising at the same 35 knots?

[CDK]

I hate to use the word 'economy' in connection with these dinosaur engines, but assuming that both sets of engines have the same peripherals, there won't be much difference in fuel consumption. If the small blocks have Rochester carbs and the large ones have injection, the gas consumption will be noticeably lower.

[Tim.M]

Thanks CDK. What's the difference in fuel economy between these dinosaur 5.7 L GM engines and a current model ~300 hp IO engine?

[Frosty]

HP is made by burning fuels. 10GPh per 100Hp for 2 stroke. 7GPH per 100HP for 4 stroke. 5 GPH per 100 HP for diesel.

Please note that is per 100 HP used.

[CDK]

Originally Posted by Tim.M

Thanks CDK. What's the difference in fuel economy between these dinosaur 5.7 L GM engines and a current model ~300 hp IO engine?

When Mecruiser adopted the EFI-system they claimed 15-20 % better fuel economy. The figures Frosty gives are rule-of-thumb, actual figures for a specific engine depend on how closely the optimal air/fuel ratio can be maintained.
With carburetors the mixture is generally on the rich side because the engine responds better, but the amount of fuel for which there is no air to burn it, is wasted. EFI offers more precise control so wastes less.

[speedboats]

A correctly tuned carb will offer the same burn rate as that of its injected brother. That is because to make 260hp you need to burn 260hp worth of fuel. Any difference will come in the way of transfer of the energy to the crank, so unless you can find an engine that wastes less energy making heat and noise when combusting fuel then they'll all be much of a muchness.

[CDK]

Originally Posted by speedboats

A correctly tuned carb will offer the same burn rate as that of its injected brother. That is because to make 260hp you need to burn 260hp worth of fuel. Any difference will come in the way of transfer of the energy to the crank, so unless you can find an engine that wastes less energy making heat and noise when combusting fuel then they'll all be much of a muchness.

True, but only for the "correctly tuned carb". That is not what you get when you buy an engine.
What you do get is a 4-barrel Rochester that offers only a mixture adjustment screw for the idle range. At 3500 rpm it provides a rich mixture which you cannot adjust, only at full throttle the air/fuel ratio is correct. Carbs have always suffered from the same problem, even the ones with 3 jets for each barrel. The best design ever was the SU constant vacuum carb with a plunger and a tapered needle, but that also had its weaknesses.
If you look at the 3D injection diagram of a modern EFI unit, it becomes clear that you can never achieve the same precision with a mechanical device.

[TeddyDiver]

Originally Posted by speedboats

A correctly tuned carb will offer the same burn rate as that of its injected brother.

Nope.. injected "brothers" have higher compression ratios and accordingly higher effiency.. like diesel engines..

[Frosty]

SU is constant velocity not constant vacuum.

carbs are adjustable all they way through rev ranges ,its just that it is done by Tapered needles on the carbs your talking about and are set by the factory.

Fuel injected engines IMHO dont offer economy over carbs in the marine engine as a constant RPM or there abouts is required. When you drive a car and release throttle to change gear for instance the fuel in injected engines is Off unlike a carb where the slow running jet is still operative.

Todays modern carbs do give almost perfect 14.7:1 and the emission control guys see to that.

Injected engines have high compression? Prove it!!

[pkoken]

Originally Posted by TeddyDiver

Nope.. injected "brothers" have higher compression ratios and accordingly higher effiency.. like diesel engines..

Higher compression gasoline engines do not have diesel efficiency.*

*Unless you use specific combinations of Marvel Mystery Oil and fuel line magnets properly aligned with the cosmic flux.

[Boston]

I made this exact conversion in my 2 1/2 ton duely cause I got a bigger horse trailer and didnt want to have to buy a bigger truck
thing sucked fuel like a prom date

didnt mater if I was hauling anything or not
it ate gas
I ended up putting a 150 gallon tank in it so I could drive past a gas station

[pkoken]

Comparing car (or truck) fuel efficiency to a boat is not a good comparison.

Car engines spend most of their time loafing along at a light partial load. Closed loop fuel injection systems can then lean the mixture WAY out, beyond stoic and back again in a process called dithering. The average fuel mixture ends up being VERY lean.

Boat engines seldom loaf along at light loading. The vast majority (almost all!) of fuel injected boat engines cannot operate in closed loop.

A perfectly tuned carburetor could equal a perfectly tuned injection system in efficiency. However the average carb is much worse than the average fuel injection system.

[Boston]

thing was a custom built 2 1/2 ton duely
the total cab weight was about 10,000 lbs and it only came out to play when I had something to haul
it was carborated because unless you get the million dollar computer to run the fuel injection perfectly your going to lean out at high altitude and burn the valves rather than end up running rich and loose some efficiency
but keeping your engine in one piece
the big trailer weighed in at about 20,000 lbs half loaded with horses and we typically were aiming for either Yellowstone or Rocky Mountain National park
so I knew the altitude change and could tune accordingly
thing never loafed along was built to the gills and you could watch the gas gauge go down when it was working up a hill
at least untill I put the big tank in it

you shouldn't use fuel injection to haul anything at the passenger level cause it leans out according to oxygen levels and not according to load
cuts the engine life in half or there about

load is a load whether its a boat hull or a trailer
just my two cents
B

[speedboats]

higher compression means more effiency? ah-huh

Higher compression means that the chemical reaction between the gas and air will happen at a faster rate (perhaps this is why we have to retard the timing... I don't know... it's just a guess). With fuel burning at a faster rate we can burn more fuel per cycle, more fuel = more power. Period.

Alot of the ECU stuff we've been doing for injected motors is open loop, this is due to the complexities of getting an O2 sensor in a water jacketed exhaust mainfold to run correctly. Not that it matters much, at throttle positions above around 30% the ECU will generally ignore the O2 sensor anyway and provide fuel according to a pre-programmed map. That map is generally conservativly rich to look after internal parts (like the piston crown and exhaust valve)

Injection has taken favour to help comply with environment standards, it messes around with the fuel ratios under certain perameters to acheive the applicable standards.

Your carb fuel mileage sucks compared to injection? Clean the carb and get it tuned, easier than grabbing your ankles to pay for an injection swap, unless you really want to make more power (more than just an extra 50hp) to make it worth it.

The effiency differences between carb and injection are negligable.

You must remember, a boat engine is always under load, to turn a prop / impellor through water will require a constant amount of power, unlike your truck that will burn power to 60, then back right off to about 8% throttle to maintan speed and overcome wind drag and rolling friction. When was the last time you 'cruised' your boat at less than 30% throttle?

CDK... you keep refering to GM product as dinosaur engines with no economy... what's your choice of engine???

[Boston]

looks like we agreed on most of it there Sr Speedy and your right about most over the road engines running at some fraction of there potential most of the time
course I was at highway most of the time and generally going up hill both ways so I think I got a handle on the fuel efficiency of the proposed swap out cause basically I did exactly what this fuel eating fiend is about to
spend more money

oh and if your carb millage is all that bad check to see if its jetted correctly during that cleaning mentioned above
there are very specific jets for very specific altitudes also there are differences between marine and otr carbs
go with a otr carb even on a marine application
its more tunable and has better filters
B